The power switch on my Audio Research D130 has failed. Now that the new (US$16.72!!) switch has arrived from ARC, I want to make sure that this never happens again. The ARC service representative told me, "...yeah, they arc and then corrode and eventually fail." Apparently, ARC has never tried putting a suppression cap across the power switch to protect it from the primary voltage spike when the switch opens. I want to put one on and I see recommendations here of about 10 nano-Farads, Class X capacitors. My problem is that caps do not receive class X or Y designations in the US. What would an equivalent dielectric type be? Some devices, such as Quencharc, have Underwriters Laboratories listing, but I think that a Quencharc (e.g. 100 nF w/ 150-ohm series resistor) is meant for DC and over-doing it a bit. Should I just grab any 630V ceramic? How high can the voltage spike from a 800W 120V primary go?

Capacitors used across AC lines are supposed to be flameproof. They aren't supposed to catch fire if they short. I have seen ceramic capacitors used in this application because ceramic doesn't burn. If you have an old computer power supply, you can salvage an X rated capacitor out of it. Some of the power supplies have 3-4 of those X capacitors in the line filter.

Don't put the cap across the switch. Put it across the primary after the switch and any fuse. X-rated caps should be available in most countries. 10nF sounds fine to me.

X caps can be made from several dielectrics. They are usually self-healing film caps, designed to cope with continuous AC stress and almost guaranteed not to fail short-circuit. Y caps are similar, but smaller values, and definitely guaranteed not to fail short-circuit - well, as guaranteed as anything can be.

Don't put the cap across the switch. Put it across the primary after the switch and any fuse... 10nF sounds fine to me.

So, you are suggesting that I use this cap as a line filter as well as a suppression cap for that moment when the amp is turned off? I suppose that putting it across the primary winding will serve both purposes, although I hadn't planned to do anything other than protect that over-priced switch.

Quote:

Originally Posted by rayfutrell

...If you have an old computer power supply, you can salvage an X rated capacitor out of it. ...

A cap on its own does not do much line filtering, although it might reduce noise a bit. Two reasons for putting it across the primary instead of across the switch:
1. When switched off, a cap across the switch would allow some mains current to flow - at the very least this could create confusion during fault tracing.
2. When switched off, a cap across the switch is exposed to full mains voltage. Assuming that domestic equipment spends more time off than on, this would shorten the cap life. Note that X caps can gradually lose their capacitance value, some more than others, because every mains spike burns away a little bit of film.

I had a .01 uf 1000 v rated ceramic cap across the power switch of my ST70 amp for 37 years. It was plugged in for 22 years then I put away for lack of US made tubes. Back online in 2011. X or Y designations were not invented in 1970 when I intalled it. I saw caps like this on Howard Sams photofacts, had a problem with turn off pop, and made a deduction which led to installing the cap. No pop at turnoff.
The power switch bridge cap I installed on the dynakit PAS2 preamp was vaporized in about 1985, but the power switch was carbon tracked across in the same event. It was turned off at the time. I came home and found the preamp on with the power switch turned off. I count that event as a surge caused by lightning, as we have a lot of that here, particularly on that day. The switches are small slide switches, nothing with a super sized gap between electrodes.
Don't go lower than 1000 v rating if you go with a ceramic cap. These were US made surplus caps from the R***** S***** grab bags of the late sixties. Modern vendors may have discovered a way to make things cheaper, or simply lie about the 1000 v rating. Buy a name brand ceramic cap, or insist on X2 which is audited by UL/CSA/VDE for a quality control program. Refrigerators/air conditioner shutoff can emphatically produce 600 v spikes on a 120 VAC line.
About the lightning, they now make MOS supressors , to go across the hot and neutral and take the surge. They are blue and have a broken S on them, and a number which is the AC RMS voltage rating. They are also UL/CSA/VDE rated. I salvage them from dead PC power supplies and put them in my amps preamps and mixer. Most of the ones I use are 500 V units from factory Variable Speed Drives. Even a slide switch should be able to hold off 500v.